Planetary eyepiece for XT8

[Bentatt]

Hi,

I recently bought an orion XT8 dobsonian (focal length: 1200, f ratio: 5.9). It came with a 25mm plossl and I bought an 11mm 82 degree explore scientific, mainly for use on small deep sky objects. I am looking to buy a planetary eyepiece. I really enjoy using the ES eyepiece so was considering buying either the 6.7mm or 4.7mm ones. 

The 6.7 doesn't seem to offer enough magnification and its focal length seems too close to my 11mm but if I bought the 4.7mm, I'd worry about its performance (someone told me that using an eyepiece with a focal length near the focal ratio of the telescope was a bad idea).

Thank you.

Ben

[John]

Hi Ben,

My view is that the 4.7mm eyepiece, giving you 255x will get used but not as much as the 6.7mm (179x) would. Ideally you would want both in your eyepiece case so you have a very high power option for good viewing conditions on targets such as Saturn, Mars, the Moon and double stars.

As the focal lengths get shorter, having eyepieces that seem close together does make sense because you tend to need more medium to high power options then you do low to medium power ones.

I have eyepieces in increments of .5 of a mm at my shortest focal lengths !

Regards,

John

[rockystar]

I think 6.7 is a good step up from 11. I've got a 6.5 ( with the same size scope as you) and I find it gives plenty of magnification, I don't think I'd get a lot of use out of an eyepiece at 4.7 ( though I do think I'll eventually get one, for those good nights, It's low on my priority list)

[bingevader]

Hello Ben!

Yep, 6.7mm will be a big step up from 11mm.

As John has said, at the smaller focal lengths, even 1mm can mean a lot more magnification.

I have a 5mm EP that gets used now and again on planets, 6mm is probably the most used, but having the 5mm and the 7mm (and the 8mm) gives you extra options depending on what the seeing conditions are like.

They are all lovely on the moon, mind you, and with a barlow on the 8mm and 7mm (I wouldn't buy a 4mm and a 3.5mm for the amount of use they'd get!) on a clear night, I've had some incredible views.

Cheers

[SpaceWalker]

IMO, one of the best planetary solutions could be a Zoom + 2x Barlow. That's what I used to do. It allows to dial proper magnification on-the-fly so you catch rear moments of stable seeing between turbulences which frequently last only 20-30 sec or even less. When you changing fix FL eyepieces you usually missing these moments.  BTW, I have the 11mm ES82 and it works pretty good combined with my 2x TeleXtender at 218x on my 1/6 Dob  when the seeing allows this magnification.

[John]

5 hours ago, SpaceWalker said:

IMO, one of the best planetary solutions could be a Zoom + 2x Barlow. That's what I used to do. It allows to dial proper magnification on-the-fly so you catch rear moments of stable seeing between turbulences which frequently last only 20-30 sec or even less. When you changing fix FL eyepieces you usually missing these moments.  BTW, I have the 11mm ES82 and it works pretty good combined with my 2x TeleXtender at 218x on my 1/6 Dob  when the seeing allows this magnification.

Yes, a zoom is very helpfull in that respect

It's also another reason for observing the same target for a while - to allow you eye to "tune in" and to catch those moments of excellent seeing.

[Charic]

On 10/26/2016 at 21:10, Bentatt said:

(someone told me that using an eyepiece with a focal length near the focal ratio of the telescope was a bad idea).

Your XT8 is very similar in specification to my Skyliner, F-1200 A-200. f/5.9 .

I use a 6mm eyepiece, the closest thing to f/5.91. This  affords me 200X  ( 1200/6=200)  ( twice the aperture for magnification! ) and your scope is no different!

Why then would you want to avoid this power when the scope is truly capable of providing x200, and more!

Under the right conditions, your telescope will have a  maximum and  a minimum limit, that best suits your eyes, and the seeing conditions,  and somewhere in-between, there's a sweetspot ?

I chose the  6mm to provide my  high power, the 32mm  provides my low power, and for my sweetspot, I chose  the 12mm. This combination works well for my eyes on an f/6 system, and if I were to only keep three eyepieces, for this scope, It would be 6,12 & 32 or I could possibly exchange the 6mm for a Barlow ,  giving  me 6mm 12mm  and 32mm? (the 32mm needs a 2" Barlow!)

These  are my guidelines  for using an 8" 200P Newtonian, gleaned from information that says  matching the focal ratio is a good starting point, then work around  from that point, until you find whats best. Your 'Best' eyepiece will be pants on some nights, as the conditions your looking through will dictate what is possible and not. My 8mm might be great one night,  an hour later, I  may need to jump to a lower powered eyepiece just to achieve the same level of detail, but sadly, the images will be smaller ( you just cant win). My guidelines are not set in stone, they just work very well.
I  use 375x with my scope ( great for my Moon views) but  only my Moon views! I doubt it would work as well on Jupiter.

[Dave In Vermont]

For viewing the planets, I want to be able to get the most detail possible in my viewing. So I usually use orthoscopic eyepieces. They may only give a 45° field-of-view, but for planetary work, that's perfectly acceptable. The only time they won't be good is for people who need to wear eyeglasses when viewing. This is due to their having a quite short amount of eye-relief. They also lend themselves to using a good Barlow on, so that a 12.5mm orthoscopic will work as a 6.25mm when I put it in my 2X TeleVue Barlow.

Have fun!

Dave